Question 16

$\textbf{Reaction P}$ $\rightarrow$ $\textbf{1}$:

The reactant is an aldoxime with a nitro group para to an ortho-bromine atom. Aqueous $NaOH$ is a strong base and a strong nucleophile. It performs two actions:

1. It causes the dehydration of the aldoxime to form a nitrile ($-CN$).

2. Due to the strong electron-withdrawing nature of the $-NO_2$ group, the ring is highly activated for Nucleophilic Aromatic Substitution ($S_NAr$). The strong nucleophile $OH^-$ displaces the $Br^-$ atom at the ortho position.

$\textbf{Major Product:}$ 2-hydroxy-5-nitrobenzonitrile (Structure 1).

$\textbf{Reaction Q}$ $\rightarrow$ $\textbf{2}$:

1. Treatment with acetic anhydride ($(CH_3CO)_2O$) converts the aldoxime into an O-acetylated oxime ($-CH=N-OAc$), turning the $-OH$ into a much better leaving group.

2. Aqueous $Na_2CO_3$ is a mild base. It facilitates an E2-type elimination of acetic acid to yield the nitrile ($-CN$). However, $Na_2CO_3$ is not a strong enough nucleophile to undergo $S_NAr$ and displace the bromine atom.

$\textbf{Major Product:}$ 2-bromo-5-nitrobenzonitrile (Structure 2).

$\textbf{Reaction R}$ $\rightarrow$ $\textbf{4}$:

The reactant is a ketoxime. When treated with aqueous $NaOH$ (strong base), the slightly acidic oxime hydroxyl group is deprotonated to form an oximate anion ($-O^-$). This oxygen acts as a powerful internal nucleophile. It attacks the ortho-carbon, completely displacing the $Br^-$ atom via an intramolecular $S_NAr$ mechanism. This cyclization results in a 5-membered isoxazole ring.

$\textit{Note: Base-catalyzed Beckmann rearrangement does not occur here.}$

$\textbf{Major Product:}$ 3-methyl-5-nitrobenzo[$d$]isoxazole (Structure 4).

$\textbf{Reaction S}$ $\rightarrow$ $\textbf{5}$:

The reactant is an O-acetylated ketoxime. When reacted with aqueous $Na_2CO_3$ (mild base), the base is only strong enough to hydrolyze the ester linkage ($-OAc$), converting it back to the original oxime ($-OH$). It is not strong enough to significantly deprotonate the oxime and drive the intramolecular cyclization seen in Reaction R. $\textbf{Major Product:}$ (E/Z)-1-(2-bromo-5-nitrophenyl)ethan-1-one oxime (Structure 5).

$\textbf{Conclusion:}$

P $\rightarrow$ 1; Q $\rightarrow$ 2; R $\rightarrow$ 4; S $\rightarrow$ 5.

Option B is correct.